The present invention relates to a method for manufacturing a multi-domain liquid crystal cell, and more particularly to a method for manufacturing a multi-domain liquid crystal cell in which the liquid crystal director is aligned by irradiating an alignment layer with light.
Liquid crystal displays, or LCDs, generally include two transparent substrates with liquid crystal material injected therebetween. The liquid crystal (LC) material typically includes anisotropic molecules, the average direction of the long axes which are referred to as the director of the LC material. The director distribution in bulk LC material is determined by its azimuthal anchoring energy of the LC molecules on the substrates and characterized, in part, by the axes of easy orientation, which correspond to the minimum surface energy of the LC material. Additional parameters determining the director distribution include the pretilt angle between the director and the substrate plane.
In order to obtain uniform brightness and high contrast ratio of the LCD, the LC molecules must be appropriately aligned or homogeneously aligned after being injected between the substrates of the display.
Alignment of the LC molecules is achieved by providing an alignment layer on the surface of the substrate. Preferably, the alignment layer includes a plurality of directional xe2x80x9cdomainsxe2x80x9d or regions having different alignment directions. If a plurality of binary domains, i.e., domains oriented in different directions, are provided on the surface of the alignment layer, a uniform viewing angle can be achieved. Both the value of the director tilt and the direction of this tilt (i.e., direction of the axis of easy orientation) are important for normal operation of LC devices having such binary, as well as multi-domain structures.
The alignment layer is typically fabricated by depositing a specially treated polymer on the surfaces of the substrates of the display. In accordance with one conventional technique, homogenous alignment is achieved by subjecting the polymer to a rubbing process to mechanically form alignment microgrooves in the polymer layer. The liquid crystal molecules are thus homogeneously or uniformly aligned due to the intermolecular interaction between the polymer of the alignment layer and the liquid crystal molecules.
In the above described rubbing process, however, defects are formed in the microgrooves which cause light scattering and random phase distortion. Moreover, dust and electrostatic discharges are produced in the alignment layer, so that the substrate is damaged and yield is decreased.
LC alignment by irradiation of photosensitive polymers with polarized UV light has been proposed as an alternative to rubbing (M. Schadt et al., Jpn. J. Appl. Phys., 31 (1992). p. 2155; T. Marusii and Yu. Reznikov et al., Mol., Master., 39, 1993, p. 161). The aligning ability of these photosensitive materials is determined by their anisotropic photo-induced properties. In the present invention, the photoalignment process is applied to create an array of domains where the easy orientation axes can possess two possible orthogonal directions.
Materials based on polyvinyl cinnamate, polysiloxane and polyamide are the most common photoaligning materials for LC displays. The directions of the easy axes in the plane of an aligning material were reported to be usually perpendicular to UV light polarized electron.
Such alignment techniques have advantages over the conventional rubbing method described above. In particular, electrostatic charges and dust are not produced on the aligning surface, as in the rubbing process. Further, by appropriate exposure of the photosensitive polymer, it is possible to control the direction of the easy orientation axis on the aligning surface and the azimuthal anchoring energy value. Further, the prescribed director distribution in an LC cell can be created.
Photoalignment techniques can also be used to generate a plurality of binary domains or a binary multi-domain structure. In one such technique described in W. Gibbon et al. (Nature, 351 (1991), p. 49), a first photosensitive substrate is rubbed unidirectionally, followed by irradiation of the substrate through a mask with polarized light to induce the easy axis perpendicular to the direction of rubbing. When the LC cell is assembled by injecting LC molecules between the first substrate and a second polymer-coated substrate which was rubbed in the same direction as the photosensitive material, the LC molecules are oriented with a 90xc2x0-twisted in regions corresponding to the transparent parts of the mask. Instead of a mask, an image formation optical system in the plane of the substrate can be used. The main drawback of this method is the necessity to use rubbing, which leads to the accumulation of dust and electrostatic charge, as well as the formation of distorted microgrooves on the aligning surfaces.
In another technique described in P. Shenon et al. (Nature, 368 (1994), p. 532), instead of rubbing the photoaligning surface, the photoalignment layer is exposed with polarized light to impart on an initial background alignment director. This method is free of the drawbacks described above, but has its own disadvantages. Namely, this method requires a double exposure of light with orthogonal polarization that requires rearrangement of the apparatus used to perform the optical exposure.
An object of the present invention is to provide a simple method for producing binary multi-domain directional alignment in an LC cell, which does not possess the drawbacks of the known methods. It is a further object of the present invention to create binary multi-domain directors in an alignment layer without any rearrangement of the optical scheme.
It has been discovered that the initial easy axis of the polymer fused in photoalignment techniques change sharply by 90xc2x0 when the intensity or dose of incident light exceeds a particular threshold.
Thus, in accordance with the present invention a method for controlling the alignment direction is provided, comprising the steps of coating a substrate with an alignment layer of a photosensitive material; irradiating the alignment layer with a first energy dose of light to impart a first alignment direction; irradiating the alignment layer with a second energy dose of light to impart a second alignment direction, the second alignment direction being perpendicular to the first alignment direction.
In addition, the method for fabricating a multi-domain LC cell using the substrate made from the above method comprises the steps of providing a first substrate and a second substrate, the first substrate is covered with a first alignment layer and the second substrate is covered with a second alignment layer; irradiating the first and second alignment layers with light to impart different alignment directions depending upon the light energy dose absorbed in each domain; assembling a cell from two substrates where the alignment layers face one another; and injecting LC material between the first and second substrates. Control of the energy dose absorbed in each domain can be achieved by varying the radiation intensity or duration.
According to another aspect of the present invention, the photosensitive material for the alignment layer comprises polymers illustrated in FIGS. 1-4.
The invention will be set forth in part by the detailed description that follows and, in part, will be made obvious from this description, or may be learned by practice of the invention. The objectives and advantages of the invention will be realized and attained by means of the actions action and their combinations pointed out in the appended claims.